Trivia question: How many plug-in hybrids have been made?

You've probably read quite a bit about plug-in hybrids. Plug-ins contain more battery cells than regular hybrids and thus can get between 70 to 100 miles to the gallon.

Converting your Prius to a plug-in reduces your gas costs to around $1 a gallon and results in far fewer emissions in most cases.. They can even be used to send electricity to the grid and prevent a brown-out.

So how many are there on the road? 50, according to Felix Kramer, president of Calcars, which promotes the concept and is trying to persuade lawmakers to pass tax breaks for plug-ins. Last month, there were only about 36 plug-ins but Google employees have converted a bunch since then.

The catch? It's not cheap to convert a Prius to a plug-in. The operation costs around $15,000, according to a representative from Hymotion, which peforms hybrid conversions. A substantial portion of the cost goes to the battery. At that level, it's very difficult to justify converting a regular Prius to a plug-in in terms of dollars and cents.

The price, though, will likely decline. Hymotion recently got bought by lithium-ion battery maker A123 systems. Combining a battery company and an installer company will likely reduce the price. General Motors and Toyota have both said that plug-in hybrids are a major priority for them. Plug-ins that come off the production line will, ideally, only cost about $4,000 more than regular hybrids. That should drive sales.

And in case you were wondering, you don't lose trunk space in a plug-in. The extra batteries go in the wheel well. We went for a ride in Felix's car. It's zippy.